Add wget_strscpy()

• include/wget/wget.h: Add wget_strscpy()
• libwget/strscpy.c: New file
• libwget/Makefile.am: Add strscpy.c
• libwget/http.c: Use wget_strscpy() instead of wget_strlcpy()
• libwget/iri.c: Likewise
• libwget/metalink.c: Likewise
• libwget/ssl_gnutls.c: Likewise
• libwget/test_linking.c: Likewise
• src/job.c: Likewise
• src/plugin.c: Likewise
• src/wget.c: Likewise

Fixes #307 (closed)